It is well known that a powder coating is less volatile and malodorous than a solvent coating and thus is very useful from the standpoint of environmental protection and regulation.
Powder coatings which have heretofore been commercially available have an average particle size of about 30 .mu.m and a broad particle size distribution because of rough classification.
On the other hand, the market demands the improvement of leveling, the reduction of the film thickness that leads to the enhancement of the working efficiency, the reduction of the total cost, etc.
The film obtained with the prior art powder coating cannot exhibit a good leveling unless it consists of two or more particle layers. Thus, the thin layer of the prior art powder coating is about 50 .mu.m at minimum. Further, since the prior art powder coating has a broad particle diameter distribution, it can easily cause roughness on the film surface due to coarse particles. Thus, no prior art powder coatings can meet the foregoing demands yet.
Further, when an electrostatic apparatus, which is dominant in the powder coating process, is used for coating, the surface to be coated attracts highly chargeable particles (those having a large particle diameter) more easily than others. The unapplied and recycled fractions of the powder coating quite differ in properties from the original product. When re-used, the fractions thus recycled cause a drop of coating efficiency and film properties.